Hitherto, as a type of steam turbine, a steam turbine which includes a plurality of stages each of which has a casing, a shaft body (rotor) rotatably provided inside the casing, stationary blades arranged to be fixed to the inner peripheral portion of the casing, and rotating blades provided in the radial forms to a shaft body on the downstream side of the stationary blades, has been known. Among such steam turbines, in the case of an impulse turbine, the pressure energy of the steam is converted into the velocity energy by the stationary blades, and the velocity energy is converted into rotational energy (mechanical energy) by the rotating blades. In addition, in the case of a reaction turbine, pressure energy is converted into velocity energy even in the rotating blades, and the velocity energy is converted into rotational energy (mechanical energy) by the reaction force of the ejected steam.
In such a type of steam turbine, there are many cases that gaps are formed in the radial direction between the tip end portion of the rotating blade and the casing that surrounds the rotating blade and forms the flow path of the steam, and gaps are also formed in the radial direction between the tip end portion of the stationary blade and the shaft body.
However, leakage steam passing through the gap at the tip end portion of the rotating blade to the downstream side does not impart rotational force to the rotating blade. In addition, leakage steam passing through the gap at the tip end portion of the stationary blade on the downstream side is not used by the stationary blade to convert pressure energy into velocity energy. Therefore, rotating force is rarely imparted to the rotating blade on the downstream side. Therefore, in order to enhance the performance of the steam turbine, the amount of leakage steam passing through the gaps needs to be reduced.
Here, a structure has been proposed in which a stepped portion of which the height is gradually increased from the upstream side toward the downstream side in the axial direction is provided at the tip end portion of a rotating blade and seal fins having gaps for the stepped portion are provided to a casing (for example, refer to Patent Document 1).
With this configuration, leakage flow passing through the gaps of the seal fins collides with the end edge portion (edge portion) forming the uneven surface of the stepped portion, so that flow resistance is increased. Therefore, the leakage flow rate passing through the gaps between the casing and the tip end portion of the rotating blade is reduced.